Oilfield chemical applications require robust surfactants that can provide good foaming at elevated temperatures and/or pressures. Suitable surfactants for this purpose include salts of alpha-olefin sulfonic dimer acid (“AOS dimer acid,” see, e.g., U.S. Pat. Nos. 4,556,107; 4,567,232; 4,607,700; 4,957,646; and 5,052,487). Dilute blends of alpha-olefin sulfonates and unsaturated fatty acids (e.g., oleic acid) have been used as steam foaming agents (see, e.g., U.S. Pat. No. 5,279,367).
Dimerization of alpha-olefin sulfonic acid (AOS acid) is described, for example, in U.S. Pat. Nos. 3,721,707 and 3,951,823. Briefly, AOS acid produced by sulfonation of one or more alpha-olefins, is heated at 110° C. to 200° C. to induce oligomerization. Under these conditions, intermediate sultones and alkene sulfonic acids are converted to alkane sulfonic acids and other products. The molecular weight of the product is roughly double that of the AOS acid, and hence the term “AOS dimer acid” to describe it. However, the structure of the product can be rather complex, as illustrated in the '707 patent.
In practice, dimerization of AOS acid is complicated by the formation of undesirable by-products, particularly hydrogen sulfide and elemental sulfur. Correspondingly, the sulfonic acid content of the AOS dimer acid is substantially lower than the theoretical amount based on the molecular weight of the olefin and the SO3 stoichiometry used to produce the AOS acid. For instance, when a 65:35 (wt./wt.) mixture of C14/C16 alpha-olefins is sulfonated with a 5% molar excess of SO3 by falling-film sulfonation, and the AOS acid is dimerized according to known methods, the resulting AOS dimer acid typically contains 2.9-3.4 meq/g of sulfonic acid content instead of a theoretical yield of 3.62 meq/g. The problems of low sulfonic acid content and generation of reduced sulfur compounds apparently have not been previously recognized or addressed.
Hydrogen sulfide is highly toxic, has a relatively low explosive limit in air, and has an offensive odor. Elemental sulfur sublimes and can deposit onto reactor and pipe surfaces, leading to equipment fouling during production and use of AOS dimer acid. Neutralization of AOS dimer acid can convert hydrogen sulfide to sodium sulfide (Na2S); however, this material can become a safety hazard later when the neutralized AOS dimer acid is acidified and H2S is liberated. It is therefore highly desirable to minimize or eliminate hydrogen sulfide and elemental sulfur during the production of AOS dimer acid.
AOS acids, upon aging, will generate some sulfur dioxide. Moreover, when AOS acids are heated to produce AOS dimer acids, the sulfur dioxide content can increase dramatically. The presence of SO2 is easily confirmed by Dräger tube analysis of the reactor headspace during AOS acid dimerization. Although prior-art processes for making AOS dimer acid generally ignore any increased generation of sulfur dioxide, we believe sulfur dioxide can be reduced to elemental sulfur, most likely via a reaction with hydrogen sulfide. Consequently, removal of sulfur dioxide and/or hydrogen sulfide during AOS dimer production merits due consideration.
The oilfield chemicals industry would benefit from improved AOS dimer acids and their salts and ways to produce them. The AOS dimer acid salts are surfactants with desirable elevated temperature foaming character. Especially needed are methods that would afford AOS dimer acids with a reduced level of undesirable by-products, particularly hydrogen sulfide and elemental sulfur. Ideally, the methods could be easily implemented without a need for substantial capital investment.